Articulated booms having a vertically movable platform are mounted on trucks and the like. These articulated booms must be provisioned for stable operation of the booms in order that the booms, and the weight they carry, do not cause overbalancing of the vehicle, and also to prevent positioning of either boom in such a location as to make it unstable and unsafe for the occupant of a basket located at the free end of the upper one of the articulated booms.
There are controls available which have an overriding effect on the power actuators associated with the upper and lower booms respectively which are intended to prevent the boom and its associated platform from approaching an unstable position when the basket is brought forward or into an over center position. The unstable position for the upper boom is the position in which the upper boom is either vertical in reference to the horizon, or nearly vertical. At this position, the articulated boom system is unstable and should the upper boom move beyond center, or into an "over center " position relative to the lower boom, the weight in the basket is most likely to cause an overbalancing of the truck.
The systems which have been proposed for correcting perpendicularity of the upper boom and an over center movement of the upper boom, are for the most part based on the concept of disabling the hydraulic system to prevent further articulated boom actuation and the booms are then returned to a stable position by controls at ground level. While this is at least a partial answer to the problem, it is still not wholly satisfactory since, what is needed, is to prevent the upper boom from moving into the unstable position in the first instance, by providing an overriding actuator means which will prevent the operator from using the controls at the basket level to effect unstable boom positions. Moreover, if by inadvertence or lack of skill, the operator does use the basket controls for actuating the basket and there results an unstable position for the upper boom, the controls at the basket level should remain operative at least to the extent of obtaining the necessary correctional movement of the booms to bring them back into a stable condition.